A modulation technique that lends itself well to digital communications is the so-called IQ modulation. Here, “I” denotes the so-called “In-phase” component of a waveform, and “Q” denotes the so-called “Quadrature” component. IQ modulation can be performed using IQ modulators.
An IQ modulator is a critical component in the signal chain for digital transmitters. IQ modulators perform the frequency translation that mixes a baseband signal to a desired location in the Radio Frequency (RF) spectrum. An IQ modulator typically comprises a Local Oscillator (LO) input that is split into In-phase (I) and Quadrature (Q) components which are separated by 90°. These two signals may drive separate mixers that are also driven by I- and Q-baseband signals. The outputs from both mixers are then summed to provide a modulated carrier either at RF or Intermediate Frequency (IF).
In radar systems, for example, such as Multiple-Input Multiple-Output (MIMO) Frequency-Modulated Continuous-Wave (FMCW) radar systems, an IQ modulator may be used as Single Side-Band (SSB) mixer for an up or down conversion of the LO signal. A complex sinusoidal signal which may be generated in baseband may be applied as control signal of the IQ modulator in order to shift the LO signal in frequency for an arbitrary value. In this way multiple radar transmitters can be activated at the same time. Such a radar system may be referred to as Frequency-Division Multiple-Access (FDMA) FMCW MIMO radar.
The performance of the IQ modulator influences the overall performance of communications systems such as radar systems. Because of non-ideal behavior of the IQ modulator there is the need for calibration. Model based approaches for the calibration of an IQ modulator are well known but have their disadvantages.
It is therefore desirable to provide improved techniques for calibrating IQ modulators.